Pulse light source

ABSTRACT

The present invention relates to a pulse light source which has a MOPA structure using a directly-modulated semiconductor laser as a seed light source, and is easily capable of outputting pulse light with a pulse width of a sub-nanosecond. The pulse light source comprises a seed light source, a first YbDF (Yb-Doped Fiber), a band-pass filter, a second YbDF, and a third YbDF, and has the MOPA structure. The band-pass filter inputs pulse light which is outputted from the seed light source and amplified by the first stage YbDF, and outputs, while separating a wavelength band of the inputted pulse light into a shorter wavelength side and a longer wavelength side with reference to a peak wavelength of the inputted pulse light, the attenuated pulse light after attenuating the optical power on one more than that on the other of the shorter wavelength side and the longer wavelength side. The second YbDF and the third YbDF amplify the pulse light outputted from the band-pass filter and output the amplified pulse light.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a pulse light source and a pulsecompression method.

2. Related Background Art

A pulse light source is used for industrial applications represented bymachining and the like, and has a trend that a higher power output and ashorter pulse width are desired. In particular, the pulse light sourceused in a laser beam machine for micro-machining is desired to have ahigher peak value and also a narrower pulse width for reducing a heateffect to an object to be machined as much as possible.

A Q switch, a mode lock, or the like has been proposed as a structurefor generating pulse light in a gas laser light source or a solid laserlight source (see Non-patent document 1). Further, a gain switchingmethod using a semiconductor laser also attracts attention as a simplermethod. The gain switching is realized after all by direct modulation ofthe semiconductor laser, and thereby, a pulse repetition rate thereof isnot restricted by a hard structure as in the mode lock and does not needan expensive component such as an acousto-optic switch, which consumeshuge electric power, as in the Q switch.

However, the semiconductor laser has a low optical output power ingeneral compared to conventional laser light sources such as the gaslaser light source and the solid laser light source, and is usedgenerally for communication or measurement. Therefore, the semiconductorlaser has not been required to have a high pulse peak power (seeNon-patent documents 1 and 2).

Non-patent document 1: Cho-kosoku hikari gijutsu (Ultra high speed lighttechnology), 2nd part, published by Maruzen in Mar. 15, 1990.

Non-patent document 2: M Kakui, et al., Optical Fiber Technology, vol.1, pp. 312-317, 1995.

Non-patent document 3: F. D. Teodoro, et al., PhotonicWest 2005.

Non-patent document 4: J. Limpert et al., Optics Express, vol. 11, p.3332, 2003.

SUMMARY OF THE INVENTION

The present inventors have examined conventional pulse light sources,and as a result, have discovered the following problems. That is,recently, there is observed sometimes a case of using a pulse lightsource having a MOPA (Maser Oscillator Power Amplifier) structure, whichcombines a directly-modulated semiconductor laser and an opticalamplifier (specifically, optical fiber amplifier), for an applicationrequiring a high power of more than 1 kW such as laser light machining(see Non-patent document 3). In such a case, the semiconductor laser isdesired to have a higher pulse peak of output light as much as possiblefor reducing a gain required to the optical fiber amplifier part. Thatis, amplitude of a modulation current is desired to be larger.

However, it is not easy to modulate a current of several hundredmilliamperes, and a rise time and a fall time thereof are limited to bereduced to several nanoseconds at a minimum (see Non-patent document 3).Meanwhile, there is a demand for a pulse width of less than 1 nsdepending on an application, and some cases require a pulse width of anorder of a femtosecond, for example (see Non-patent document 4).However, for generating the femtosecond pulse light, it is necessary touse a special optical amplification technique such as CPA or the like,and, in addition, there is a problem that pulse energy thereof is smalland throughput in the laser machining is low.

From the above problems, the inventors consider that it has beendifficult for the current fiber laser technology to reduce the pulsewidth on the time axis using a simple method without increasing a widthon the frequency axis.

The present invention has been developed to eliminate the problemsdescribed above. It is an object of the present invention to provide apulse light source and a pulse compression method that can easily outputpulse light with a sub-nanosecond pulse width, the pulse light sourcehaving the MOPA structure using, as a seed light source, a semiconductorlaser modulated directly with a modulation amplitude of exceeding 200mA.

A pulse light source according to the present invention comprises asemiconductor laser, a first optical filter, and an optical amplifier.The semiconductor laser is a laser capable of direct modulation andoutputs pulse light. The first optical filter inputs the pulse lightoutputted from the semiconductor laser, and outputs, while separating awavelength band of the inputted pulse light into a shorter wavelengthside and a longer wavelength side with reference to a peak wavelength ofthe inputted pulse light, the attenuated pulse light after attenuatingthe optical power on one more than that on the other of the shorterwavelength side and the longer wavelength side. The optical amplifierincludes a predetermined optical amplification medium and amplifies thepulse light outputted from the first optical filter. In the pulse lightsource having the MOPA structure, the pulse light outputted from thedirectly-modulated semiconductor laser is amplified by the opticalamplifier, after having been attenuated on one more than the other ofthe shorter wavelength side and the longer wavelength side of thewavelength band of the pulse light by the first optical filter.

In the pulse light source according to the present invention, the firstoptical filter preferably outputs the pulse light after havingattenuated its inputted light components on the shorter wavelength sidethan the longer wavelength side of the wavelength band of the pulselight with reference to the peak wavelength of the pulse light. Thefirst optical filter preferably has variable transmittancecharacteristics. Further, the pulse light source preferably furthercomprises a second optical filter disposed so as to sandwich the opticalamplification medium included in the optical amplifier with the firstoptical filter. One of the first optical filter and the second opticalfilter is preferably a band-pass filter. Both of the first opticalfilter and the second optical filter may be the band-pass filters. Atthis time, a full width at half maximum of a transmission spectrum inthe second optical filter located on the down-stream side of the opticalamplification medium is preferably wider than a full width at halfmaximum of a transmission spectrum of the first optical filter locatedon the up-stream side of the optical amplification medium. Further, whenboth of the first optical filter and the second optical filter are theband-pass filters, the center wavelength in the transmission filter ofthe second optical filter may be set between the peak wavelength of thepulse light and the center wavelength in the transmission filter of thefirst optical filter. The semiconductor laser is preferably aFabry-Perot type semiconductor laser. The semiconductor laser isprovided with a temperature adjustment means adjusting the temperaturethereof. The pulse light outputted from the optical amplifier preferablyhas a pulse width of less than 1 ns. The pulse light outputted from theoptical amplifier preferably has a peak power of exceeding 1 kW. Thepulse light preferably has a peak power of exceeding 10 kW in the casethat a repetition frequency is 1 MHz.

A pulse compression method according to the present invention performspulse compression by utilizing the pulse light source (pulse lightsource according to the present invention) which comprises thesemiconductor laser, the first optical filter, and the opticalamplifier, as described above. In particular, the pulse compressionmethod according to the present invention attenuates the inputted lightcomponents on one more than the other of the shorter wavelength side andthe longer wavelength side of the output spectrum of the semiconductorlaser with reference to the peak wavelength of the pulse light, by thepulse light source adjusting a relative positional relationship betweenthe transmission wavelength band of the optical filter and the outputspectrum of the semiconductor laser.

In the pulse compression method according to the present invention, thefirst optical filter preferably outputs, while separating a wavelengthband of the inputted pulse light into a shorter wavelength side and alonger wavelength side with reference to a peak wavelength of theinputted pulse light, the attenuated pulse light after attenuating theoptical power on the shorter wavelength side more than that on thelonger wavelength side. The first optical filter preferably has thevariable transmission characteristics. The second optical filter may bedisposed so as to sandwich the optical amplification medium included inthe optical amplifier with the first optical filter. At least one of thefirst optical filter and the second optical filter is preferably theband-pass filter. Both of the first optical filter and the secondoptical filter may be the band-pass filters. In this case, the fullwidth at half maximum of the transmission spectrum in the second opticalfilter is preferably wider than the full width at half maximum of thetransmission spectrum in the first optical filter. Further, when both ofthe first optical filter and the second optical filter are the band-passfilters, the center wavelength in the transmission filter of the secondoptical filter may be set between the peak wavelength of the pulse lightand the center wavelength in the transmission filter of the firstoptical filter. The semiconductor laser is preferably the Fabry-Perottype. Further, the semiconductor laser is preferably provided with thetemperature adjustment means adjusting temperature thereof.

The present invention will be more fully understood from the detaileddescription given hereinbelow and the accompanying drawings, which aregiven by way of illustration only and are not to be considered aslimiting the present invention.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the scope of the invention will be apparent tothose skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a configuration of a first embodiment of a pulse lightsource according to the present invention;

FIG. 2 is a graph showing a modulated voltage waveform in directmodulation of a semiconductor laser;

FIG. 3 is a graph showing a time waveform of pulse light outputted froma semiconductor laser;

FIG. 4A is a graph showing a time waveform of pulse light outputted froma semiconductor laser, and FIG. 4B is a graph showing a time change ofchirping in pulse light outputted from a semiconductor laser;

FIG. 5 is a graph showing time waveforms of pulse light outputted fromthe pulse light source according to the first embodiment;

FIG. 6 is a graph showing time waveforms of pulse light outputted fromthe pulse light source according to the first embodiment;

FIGS. 7A to 7C show pulse waveforms and spectra when the pulse lightoutputted from the seed light source is modified by adjustment of thecenter wavelength of the band-pass filter provided in the subsequentstage of the seed light source;

FIG. 8 shows a configuration of a second embodiment of a pulse lightsource according to the present invention;

FIGS. 9A and 9B are graphs showing exemplarily a state of ASE removal bythe band-pass filters in the pulse light source according to the secondembodiment;

FIGS. 10A to 10C are graphs showing exemplarily a state of ASE removalby the band-pass filters in the pulse light source according to thesecond embodiment;

FIG. 11 shows a configuration of the ASE light source using the YbDF orthe YbDF;

FIG. 12 shows an output light spectrum of the ASE light source using theYbDF or the YbDF;

FIG. 13 is a graph showing pulse waveforms of the pulse light outputtedfrom the pulse light source according to the second embodiment; and

FIG. 14 is a graph showing a relationship between a repetition frequencyand a pulse peak power of the pulse light outputted from the pulse lightsource according to the second embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, embodiments of a pulse light source and a pulsecompression method according to the present invention will be explainedin detail with reference to FIGS. 1-4, 4A, 4B, 5-6, 7A-7C, 8, 9A-10C,and 11-14. In the description of the drawings, identical orcorresponding components are designated by the same reference numerals,and overlapping description is omitted.

FIG. 1 shows a configuration of a first embodiment of a pulse lightsource according to the present invention. The pulse light source 1, asshown in FIG. 1, comprises a seed light source 10, an YbDF (Yb-DopedFiber) 20, a band-pass filter 30, and YbDFs 40 and 50, and so on, andhas the MOPA structure. This pulse light source 1 outputs pulse lighthaving a wavelength around 1060 nm which is suitable for lasermachining.

The seed light source 10 includes a semiconductor laser which isdirectly modulated and outputs pulse light. This semiconductor laser ispreferred to be a Fabry-Perot type, from a viewpoint of achieving higherpower and also from a viewpoint of avoiding non-linear effects such asstimulated Brillouin scattering (SBS). Further, the semiconductor laseroutputs pulse light with the wavelength of around 1060 nm, at which theamplification optical fibers of the YbDFs 20, 40, and 50 have gains. TheYbDFs 20, 40, and 50 are optical fibers which is mainly composed ofsilica glass and whose core is doped with Yb element as an activator.They have an advantage that the wavelengths of pumping light and lightto be amplified are close to each other and a high power conversionefficiency is obtained, and an advantage that a high gain is obtained inthe vicinity of the wavelength 1060 nm. These YbDFs 20, 40, and 50constitute a three-stage optical fiber amplifier.

The first stage YbDF 20 is provided with forward direction supply of thepumping light which is outputted from a pumping light source 22 andtransmitted through an optical coupler 21. Then, the YbDF 20 inputs thepulse light which is outputted from the seed light source 10 andtransmitted through an optical isolator 23 and the optical coupler 21,and amplifies the inputted pulse light and outputs the amplified pulselight via an optical isolator 24.

The band-pass filter 30 inputs the pulse light which is outputted fromthe seed light source 10 and amplified by the first stage YbDF 20, andoutputs, while separating a wavelength band of the inputted pulse lightinto a shorter wavelength side and a longer wavelength side withreference to a peak wavelength of the inputted pulse light, theattenuated pulse light after attenuating the optical power on one morethan that on the other of the shorter wavelength side and the longerwavelength side. Note that a high-pass filter or a low-pass filter maybe used instead of the band-pass filter, but the high-pass filter cancut out only the longer wavelength side of the seed light sourcespectrum and the low-pass filter can cut out only the shorter wavelengthside of the seed light source spectrum. The band-pass filter has theboth functions and is preferable for this purpose.

The second stage YbDF 40 is provided with forward direction supply ofthe pumping light which is outputted from a pumping light source 42 andtransmitted through an optical coupler 41. Then, the YbDF 40 inputs thepulse light which is outputted from the band-pass filter 30 andtransmitted through an optical isolator 43 and the optical coupler 41,and amplifies the inputted pulse light and outputs the amplified pulselight via an optical isolator 44. The third stage YbDF 50 is providedwith forward direction supply of pumping the light which is outputtedfrom each of pumping light sources 52 and 55 and transmitted through acombiner 51. Then, the YbDF 50 inputs the pulse light amplified by thesecond stage YbDF 40, and further amplifies the inputted pulse light andoutputs the amplified pulse light to outside via an end gap 60.

A more preferable configuration example is as follows. The first stageYbDF 20 uses a core pumping method, and inputs the pumping light, whichhas a constant power of 200 mW at a pumping wavelength of 975 nm, in theforward direction. The YbDF 20 uses a 5 m long optical fiber having anunsaturated absorption coefficient of 240 dB/m at the wavelength of 975nm. The core of the YbDF20 has a diameter of 6 μm and a NA ofapproximately 0.12. The second stage YbDF 40 uses the core pumpingmethod, and inputs the pumping light, which has a constant power of 200mW at the pumping wavelength of 975 nm, in the forward direction. TheYbDF 40 uses a 7 m long optical fiber having an unsaturated absorptioncoefficient of 240 dB/m at the wavelength of 975 nm. The core of theYbDF40 has a diameter of 6 μm and a NA of approximately 0.12. The thirdYbDF 50 uses a cladding-pumping method, and inputs the pumping lights,which have a power of 20 W (four pumping LDs of a 5 W class) at thepumping wavelength of 975 nm, in the forward direction. The YbDF 50 usesa 5 m long optical fiber having an unsaturated absorption coefficient inthe core of 1200 dB/m. The core of the YbDF 50 has a diameter of 10 μmand a NA of approximately 0.06. The inner cladding of the YbDF 50 has anouter diameter of 125 μm and a NA of approximately 0.46.

FIG. 2 is a graph showing a modulation voltage waveform for the directmodulation of a semiconductor laser. Further, FIG. 3 is a graph showinga time waveform of pulse light outputted from the semiconductor laser.In FIG. 2, the repletion frequency is set to be 100 kHz. The maximummodulated current amplitude of 240 mA is obtained for the rise time andfall time of the modulated voltage waveform shown in FIG. 2. In the timewaveform of the pulse light outputted from the semiconductor laser whichis provided with this modulation signal, an over shoot half width isless than 1 ns as shown in FIG. 3, but a pulse component with a widthexceeding 10 ns exists following the overshoot and this pulse componentcauses the heat effect in the laser machining. Meanwhile, it isdifficult to keep the current amplitude of 240 mA when the modulationsignal width is reduced to be less than 1 ns.

In the case that the semiconductor laser has a single wavelengthoscillation such as in DFB or the like, a relationship between opticaloutput power P and optical frequency change Δv (so called chirping) ofthe oscillation is given by the following Formulas 1a and 1b. Here, α isa line width increase coefficient, Γ is a confinement coefficient of anactive layer, ε is a non-linear gain coefficient, V is an active layervolume, h is the Planck's constant, and v is an optical frequency of aseed light source output.

$\begin{matrix}{{\Delta \; v} = {\frac{a}{4\; \pi}\left( {{\frac{1}{P} \cdot \frac{\partial P}{\partial t}} + {\kappa \; P}} \right)}} & \left( {1\; a} \right) \\{\kappa = \frac{2\; \Gamma \; ɛ}{Vhv}} & \left( {1\; b} \right)\end{matrix}$

FIG. 4A is a graph showing a time waveform of the pulse light outputtedfrom the semiconductor laser, and FIG. 4B is a graph showing time changeof the chirping in the pulse light outputted from the semiconductorlaser. As apparent from the above Formulas 1a and 1b, the chirping Δvdepends on a time derivation of the optical output power P. Therefore,as shown in FIGS. 4A and 4B, the approximately maximum chirping occursat the largest change of the optical output power P. Then, since themost distinguished change of the optical output occurs at the overshootof the rising, the chirping occurs generally at a high frequency (i.e.,on the shorter wavelength side) and has a spectrum shape with a longtail in the short wavelength side even in the semiconductor laser of theFabry-Perot type.

Here, when the center wavelength of the band-pass filter 30 included inthe pulse light source 1 shown in FIG. 1 is shifted intentionally fromthe maximum intensity wavelength of the output light spectrum of theseed light source 10 toward the shorter wavelength side or the longerwavelength side thereof, only the chirping component shown in FIG. 4 canbe cut out and it is possible to remove the pulse continuing in morethan 10 ns after the overshoot as shown in FIG. 3. Note that this methodobviously cuts most of the output power of the seed light source 10 bythe band-pass filter 30, and therefore the YbDFs 20, 40, and 50 areprovided for compensating this output cut. The optical fiber amplifierpart including these YbDFs 20, 40, and 50 has a potential to provide asufficient gain and can realize a gain of several tens of dB.

FIG. 5 is a graph showing the time waveform of the pulse light outputtedfrom the pulse light source 1 according to the first embodiment. Here,by use of a band-pass filter, in which the transmittance wavelength bandis variable, the transmittance wavelength band of the band-pass filter20 is adjusted. In FIG. 5, the time waveform A1 is measured when thetransmittance wavelength band of the band-pass filter 20 is shifted tothe longer wavelength side as much as possible and also the pulse peakis maximized. The time waveform A2 is measured when the transmittancewavelength of the band-pass filter 20 is shifted to the shorterwavelength side as much as possible and also the pulse peak ismaximized. Further, the time waveform A3 is measured when thetransmittance waveform of the band-pass filter 20 overlaps the outputspectrum of the seed light source 10 as precisely as possible.

As shown in FIG. 5, the pulse peak even increases by the effect of theoptical fiber amplifier part of the YbDFs 20, 40, and 50. Note that,when the center wavelength of the band-pass filter 30 is shifted toomuch against the seed light source spectrum, the light obviously cannottransmit the band-pass filter 30. The time waveforms A1 and A2 in FIG. 5are obtained when the transmittance wavelength band of the band-passfilter 30 is adjusted to the longer wavelength side or the shorterwavelength side, respectively, such that the pulse peak is maximized.Then, when the center wavelength in the transmission wavelength band ofthe band-pass filter 30 is shifted to fit the longer wavelength side ofthe seed light source spectrum, an approximately 10% higher pulse peakand a sharper pulse fall are obtained than in the case in which thecenter wavelength is shifted to fit the shorter wavelength side, and theheat effect is expected to be suppressed in the laser machining.

Note that, while the semiconductor laser has largely-distributedtransient response characteristics among samples, the result shown inFIG. 6 is obtained even when the semiconductor laser, which does nothave overshoot at all, is used for the seed light source 10, and apreferable result is obtained there also when the center wavelength ofthe band-pass filter 30 is shifted to fit the longer wavelength side ofthe seed light source spectrum. FIG. 6 is a graph showing the timewaveform of the pulse light outputted from the pulse light source 1 whenthe semiconductor laser, which does not have overshoot at all, is usedfor the seed light source 10. In FIG. 6, the time waveform B1 isobtained when the transmittance wavelength band of the band-pass filter20 is shifted to the longer wavelength side as much as possible and alsothe pulse peak is maximized. The time waveforms B2 to B6 are measuredwhen the transmittance wavelength band of the band-pass filter 20 isshifted gradually to the shorter wavelength side from the position forthe time waveform B1. As apparent from FIG. 6, a preferable result isobtained when the center wavelength of the band-pass filter 30 isshifted to fit the longer wavelength side of the seed light sourcespectrum.

The above description explains the method to make the transmittancespectrum of the band-pass filter 30 to be variable for adjusting therelative positional relationship between the seed light source spectrumand the transmission spectrum of the band-pass filter 30, but almost thesame effect can be expected to be obtained by adjustment of thetemperature in the seed light source 10. While the adjustment of theband-pass filter 30 requires mechanical adjustment such as angleadjustment of a dielectric multilayer film, the temperature adjustmentof the seed light source 10 can be carried out only by electroniccontrol and has advantages in reproducibility and controllability.

As described above, the pulse light source 1 according to the firstembodiment is a pulse light source, which has the MOPA structure usingthe seed light source of the semiconductor laser modulated directly witha modulation amplitude of exceeding 200 mA, and can easily output pulselight having a pulse width with a sub-nanosecond. In particular, thepulse light source 1 according to the first embodiment can output thepulse light having a peak power of exceeding 1 kW and a pulse width ofless than 1 ns, and therefore the pulse light source 1 is preferable forapplication to the laser machining. Further, the pulse light source 1according to the present embodiment directly modulates the semiconductorlaser included in the seed light source 10 and has a high degree offreedom in the modulation compared to the mode lock.

Meanwhile, the pulse light source 1 in the configuration shown in FIG. 1uses the Fabry-Perot type semiconductor laser as the seed light source10. For realizing the short pulse, as shown in FIGS. 7A and 7B, thecenter wavelength of the band-pass filter 30, which is provided in thesubsequent stage of the seed light source 10, is adjusted so as toobtain the state of C2 or C3 in the drawings, and thereby the full widthat half maximum of the pulse can be compressed approximately from 5 nsto 0.5 ns.

FIG. 7A shows a pulse waveform in the case of modifying the pulse lightoutputted from the seed light source 10 by adjusting the centerwavelength of the band-pass filter 30 provided in the subsequent stageof the seed light source 10. FIG. 7B shows the spectrum in this case.Further, FIG. 7C shows an enlarged part of FIG. 7A. The plot C1 in thedrawing shows a case without the band-pass filter. The plots C2 to C7show cases of shifting the center wavelength of the band-pass filtergradually from the longer wavelength side to the shorter wavelengthside.

Note that, when the center wavelength of the band-pass filter 30 isde-tuned largely from the center wavelength of the seed light sourcespectrum as in the cases of the plots C2 and C3, ASE generated in theYbDF in the downstream increases. For suppressing such an ASE component,a plurality of band-pass filters are preferably inserted inside theoptical amplifier coupled in the downstream of the seed light source, asshown in FIG. 8.

FIG. 8 shows a configuration of a second embodiment of a pulse lightsource 2 according to the present invention. The pulse light source 2shown in this FIG. 8 comprises a seed light source 10, an YbDF 110, aband-pass filter 120, an YbDF 130, a band-pass filter 140, an YbDF 150,an YbDF 160, and so on, and has the MOPA structure. This pulse lightsource 2 outputs pulse light having a wavelength of around 1060 nm whichis suitable for the laser machining.

The YbDFs 110, 130, 150, and 160 amplify the pulse light which isoutputted from the seed light source 10 and has the wavelength around1060 nm, and are optical fibers which are comprised of glass and havethe Yb element added to the core thereof as an activator. Each of theYbDF 110, 130, 150, and 160 has a pumping light wavelength that is closeto the wavelength of light to be amplified, and has an advantage in thepoint of power conversion efficiency and also an advantage of realizinga high gain around the wavelength of 1060 nm. These YbDFs 110, 130, 150,and 160 constitute a four stage optical fiber amplifier.

The first stage YbDF 110 is provided with forward supply of pumpinglight which is outputted from a pumping light source 112 and transmittedthrough an optical coupler 113 and an optical coupler 111. Then, theYbDF 110 inputs pulse light which is outputted from the seed lightsource 10 and is transmitted through an optical isolator 114 and theoptical coupler 111, and amplifies the inputted pulse light and outputsthe amplified pulse light via an optical isolator 115.

The band-pass filter 120 inputs the pulse light which is amplified bythe first stage YbDF 110 and transmitted through the optical isolator115, and outputs, while separating a wavelength band of the inputtedpulse light into a shorter wavelength side and a longer wavelength sidewith reference to a peak wavelength of the inputted pulse light, theattenuated pulse light after attenuating the optical power on one morethan that on the other of the shorter wavelength side and the longerwavelength side.

The second stage YbDF 130 is provided with forward supply of the pumpinglight which is outputted from the pumping light source 112 andtransmitted through the optical coupler 113 and an optical coupler 131.Then, the YbDF 130 inputs the pulse light which is outputted from theband-pass filter 120 and transmitted through the optical coupler 131,and amplifies the inputted pulse light and outputs the amplified pulselight.

The band-pass filter 140 inputs the pulse light amplified by the secondstage YbDF 130, and outputs, while separating a wavelength band of theinputted pulse light into a shorter wavelength side and a longerwavelength side with reference to a peak wavelength of the inputtedpulse light, the attenuated pulse light after attenuating the opticalpower on one more than that on the other of the shorter wavelength sideand the longer wavelength side.

The third stage YbDF 150 is provided with forward supply of the pumpinglight which is outputted from a pumping light source 152 and transmittedthrough an optical coupler 151. Then, the YbDF 150 inputs the pulselight which is outputted from the band-pass filter 140 and transmittedthrough an optical isolator 153, and amplifies the inputted pulse lightand outputs the amplified pulse light.

The fourth stage YbDF 160 is provided with forward supply of the pumpinglight which is outputted from each of pumping light sources 162 to 166and transmitted through a combiner 161. Then, the YbDF 160 inputs thepulse light which is amplified by the third stage YbDF 150 andtransmitted through the optical isolator 167 and the combiner 161, andamplifies the inputted pulse light further more and outputs theamplified pulse light to outside via an end gap 60.

A more preferable configuration example is as follows. The first stageYbDF 110 is a single cladding Al-codoped silica-based YbDF, and has anAl concentration of 5 wt %, a core diameter of 10 μm, a claddingdiameter of 125 μm, a non-saturation absorption of 70 dB/m for thepumping light at a 915 nm band, a non-saturation absorption peak of 240dB/m for the pumping light at a 975 nm band, and a length of 7 m. Thesecond stage YbDF 130 is a single cladding Al-codoped silica-based YbDF,and has a Al concentration of 5 wt %, a core diameter of 10 μm, acladding diameter of 125 μm, a non-saturation absorption of 70 dB/m forthe pumping light at the 915 nm band, a non-saturation absorption peakof 240 dB/m for the pumping light at the 975 nm band, and a length of 7m.

The third stage YbDF 150 is a double-cladding phosphate-glass-basedYbDF, and has a P concentration of 26.4 wt %, an Al concentration of 0.8wt %, a core diameter of 10 μm, a first cladding diameter ofapproximately 125 μm, an octagonal shape in the cross-section of thefirst cladding, a non-saturation absorption of 1.8 dB/m for the pumpinglight at the 915 nm band, and a length of 3 m. The fourth stage YbDF 160is a double-cladding Al-codoped silica-based YbDF, and has an Alconcentration of 5 wt %, a core diameter of 10 μm, a cladding diameterof 125 μm, a non-saturation absorption of 80 dB/m for the pumping lightat the 915 nm band, and a length of 3.5 m.

All the wavelengths of the pumping lights supplied to the YbDFs 110,130, 150, and 160 are in the 0.975 μm band. The pumping light suppliedto the YbDF 110 has a power of 200 mW in a single mode. The pumpinglight supplied to the YbDF 130 has a power of 200 mW in a single mode.The pumping light supplied to the YbDF 150 has a power of 2 W in multimodes. Further, the pumping light supplied to the YbDF 160 has a powerof 14 W in multi modes.

Each of the band-pass filters 120 and 140 has a full width at halfmaximum of the transmission spectrum of 3 nm. FIGS. 9A, 9B, 10A, 10B,and 10C are diagrams showing exemplarily states of the ASE removal bythe band-pass filters 120 and 140 in the pulse light source 2 accordingto the second embodiment.

As shown in FIGS. 9A and 9B, when the center wavelength in thetransmission spectrum of the band-pass filter 120 (D1 in FIGS. 9A and9B) roughly fits the peak wavelength in the output light spectrum of theseed light source 10 (D2 in FIGS. 9A and 9B), the power of the lightoutputted from the band-pass filter 120 (D3 in FIGS. 9A and 9B) can bemaintained to be high, and an S/N ratio can be maintained to be highcompared to the ASE component contained in the light outputted from theYbDF 130 in the subsequent stage (D4 in FIGS. 9A and 9B).

On the other hand, as shown in FIGS. 10A to 10C, when the centerwavelength in the transmission spectrum of the band-pass filter 120 (E1in FIGS. 10A to 10C) is shifted largely from the peak wavelength in theoutput light spectrum of the seed light source 10 (E2 in FIGS. 10A to10C), the power of the light outputted from the band-pass filter 120 (E3in FIGS. 10A to 10C) is attenuated largely from that of the input lightand the S/N ratio is degraded considerably compared to the ASE componentcontained in the light outputted from the YbDF 130 in the subsequentstage (E4 in FIGS. 10A to 10C). For avoiding this problem, the band-passfilter 140 is inserted further in the downstream thereof, and the S/Nratio of the light outputted from the band-pass filter 120 (E5 in FIGS.10A to 10C) can be improved. Note that, at this time, the centerwavelength of the band-pass filter 140 is preferably set to be closer tothe peak wavelength in the output spectrum of the seed light source 10than the center wavelength of the band-pass filter 120.

In the pulse light source 2 shown in FIG. 8, when the seed light source10 is oscillated continuously at an output power of 20 dBm for the abovespecific configuration, the pulse light outputted from the end gap has apower of 10.7 W. When the center wavelength of the band-pass filter 120is shifted from the peak wavelength in the output spectrum of the seedlight source 10 to the longer wavelength side, as the case of the plotC2 in FIGS. 7A to 7C, a time-averaged input power of the light inputtedto the second stage YbDF 130 is reduced and the ASE outputted from theinside of the second stage YbDF 130 is increased. As a result, theoutput power of the light to be amplified against the ASE power (calledS/N ratio for convenience) is lowered immediately after the second stageYbDF 130.

By providing the band-pass filter 140 in the subsequent stage of thesecond YbDF 130 and also by using the phosphate glass YbDF, which has anarrow ASE band, for the third stage YbDF 150, it is possible to improvethe S/N ratio as shown in FIGS. 11 and 12.

FIG. 11 is a diagram showing a configuration of an ASE light source 3using the YbDF 150 or the YbDF 160. This ASE light source 3 outputspumping light with a power of 7 W from each of six pumping light sources213 to 218 and supplies this pumping light to an YbDF 210 via a combiner211. The YbDF 150 (8 m) or the YbDF 160 (10 m) is used for the YbDF 210.The combiner 211 has a pumping light input port constituted by sixmulti-mode fibers, an input port constituted by one single-mode fiberfor the light to be amplified, and an output port constituted by onedouble-cladding fiber. An isolator 219 reduces an optical back flow atthe wavelength of 1060.

FIG. 12 shows an output spectrum of the ASE light source 3 using theYbDF 150 or the YbDF 160. This FIG. 12 shows the spectrum by the plot F1when the input power of the ASE light source 3 is zero and the YbDF 150is used for the YbDF 210, and shows the spectrum by the plot F2 whenYbDF 160 is used for the YbDF 210. As shown in this FIG. 12, by usingthe phosphate-glass YbDF for the third stage YbDF 150, it is possible toimprove the S/N ratio.

FIG. 13 is a graph showing the waveform of the pulse light outputtedfrom the pulse light source 2 according to the second embodiment. InFIG. 13, the repetition frequencies are 100 kHz, 166.7 kHz, 200 kHz,312.5 kHz, 500 kHz, 1 MHz, and 2.5 MHz. FIG. 14 is a graph showing arelationship between the repetition frequency and the pulse peak powerof the pulse light outputted from the pulse light source 2 according tothe second embodiment. Note that, in the measurement of the pulsewaveform, an spatial attenuator having an attenuation of approximately65 dB was inserted after the end gap 60 located at an output end of thepulse light source 2, the light outputted from the end gap 60 wasreceived by a photo-electric conversion module made by Thorlabs (SIR5-FCtype), and the waveform of the electric output from the photo-electricconversion module was observed with an oscilloscope made by YokogawaElectric (DL9240).

As shown in FIGS. 13 and 14, it is possible to realize a pulse peak ofmore than 10 kW even at an output pulse light repetition frequency of 1MHz, and to realize a pulse peak of 56 kW at an output pulse lightrepetition frequency of 100 kHz. Note that the phosphate-glass YbDF forthe third stage YbDF 150 has a lower pumping efficiency (ratio of theoutput power of the light to be amplified obtained at a specific pumpingpower) compared to the fourth stage YbDF 160. Accordingly, theAl-codoped silica-based YbDF is used for the last stage YbDF 160 in theconfiguration shown in FIG. 8.

The center wavelength of the band-pass filter 140 is preferably set tobe closer to the peak wavelength in the output spectrum of the seedlight source 10 than the center wavelength of the band-pass filter 120.Further, while the full width at half maximum of the transmissionspectrum in each of the band-pass filters 120 and 140 has been assumedto be equally 3 nm, the spectrum of the light to be amplified after thetransmission of the band-pass filter 120 has generally a wider band thanthe transmission spectrum original to the band-pass filter 120 as aresult of an each other's shift between the peak wavelength of theoutput of the seed light source 10 and the center wavelength of theband-pass filter 120, and therefore, the full width at half maximum ofthe transmission spectrum of the band-pass filter 140 is preferablywider than the full width at half maximum of the transmission spectrumof the band-pass filter 120.

Note that the widening of the spectrum is at such a level that anoriginal full width at half maximum of 3 nm in the transmission spectrumof the band-pass filter becomes a full width at half maximum of 4 nm inthe spectrum after the transmission at a maximum as shown in the plot C2of FIG. 7B. Therefore, the full width at half maximum in thetransmission spectrum of the band-pass filter 140 is preferably about1.5 times of that in the transmission spectrum of the band-pass filter120 at a maximum.

Further, the band-pass filter is not always required to have two stagesand may have three stages. Moreover, the band-pass filter itself is notalways required and it is optional to use a combination of a shortwavelength transmission filter (SWPF) and a long wavelength transmissionfilter (LWPF).

As described above, in accordance with the pulse light source accordingto the present invention, a pulse light source, which has the MOPAstructure using, as a seed light source, a semiconductor laser directlymodulated with a modulation amplitude of more than 200 mA, can berealized and pulse light with a sub-nanosecond pulse width can be easilyoutputted.

From the invention thus described, it will be obvious that theembodiments of the invention may be varied in many ways. Such variationsare not to be regarded as a departure from the spirit and scope of theinvention, and all such modifications as would be obvious to one skilledin the art are intended for inclusion within the scope of the followingclaims.

1. A pulse light source, comprising: a semiconductor laser being capableof direct modulation and outputting pulse light; a first optical filterinputting the pulse light outputted from the semiconductor laser, andoutputting, while separating a wavelength band of the inputted pulselight into a shorter wavelength side and a longer wavelength side withreference to a peak wavelength of the inputted pulse light, theattenuated pulse light after attenuating the optical power on one morethan that on the other of the shorter wavelength side and the longerwavelength side; and an optical amplifier amplifying the pulse lightoutputted from the first optical filter and outputting the amplifiedpulse light.
 2. A pulse light source according to claim 1, wherein thefirst optical filter outputs the attenuated pulse light afterattenuating the optical power on the shorter wavelength side more thanthat on the longer wavelength side of the wavelength band of theinputted pulse light.
 3. A pulse light source according to claim 1,wherein the first optical filter has variable transmittancecharacteristics.
 4. A pulse optical source according to claim 1, furthercomprising a second optical filter disposed so as to sandwich an opticalamplification medium included in the optical amplifier with the firstoptical filter.
 5. A pulse light source according to claim 1, whereinthe first optical filter includes a band-pass filter.
 6. A pulse lightsource according to claim 4, wherein at least one of the first opticalfilter and the second optical filter includes a band-pass filter.
 7. Apulse optical source according to claim 4, wherein each of the firstoptical filter and the second optical filter includes a band-passfilter, and wherein a full width at half maximum of a transmissionspectrum in the second optical filter is wider than a full width at halfmaximum of a transmission spectrum in the first optical filter.
 8. Apulse light source according to claim 4, wherein each of the firstoptical filter and the second optical filter includes a band-passfilter, and wherein a center wavelength in a transmission spectrum ofthe second optical filter is set between the peak wavelength of thepulse light and a center wavelength in a transmission spectrum of thefirst optical filter.
 9. A pulse light source according to claim 1,wherein the semiconductor laser includes a Fabry-Perot typesemiconductor laser.
 10. A pulse light source according to claim 1,further comprising a temperature adjustment unit adjusting temperatureof the semiconductor laser.
 11. A pulse light source according to claim1, wherein the pulse light outputted from the optical amplifier has apulse width of less than 1 ns.
 12. A pulse light source according toclaim 1, wherein the pulse light outputted from the optical amplifierhas a peak power of exceeding 1 kW.
 13. A pulse light source accordingto claim 12, wherein the pulse light has a peak power of exceeding 10 kWwhen a repetition frequency is 1 MHz.
 14. A pulse compression method,comprising the steps of: preparing a pulse light source which comprises:a semiconductor laser being directly modulated and outputting pulselight; a first optical filter inputting the pulse light outputted fromthe semiconductor laser and transmitting the inputted pulse light in onepart of a wavelength band of the pulse light; and an optical amplifieramplifying the pulse light outputted from the first optical filter andoutputting the amplified pulse light, and attenuating, while separatinga wavelength band of the inputted pulse light into a shorter wavelengthside and a longer wavelength side with reference to a peak wavelength ofthe inputted pulse light, the optical power on one more than that on theother of the shorter wavelength side and the longer wavelength side, bythe prepared pulse light source adjusting a relative positionalrelationship between a transmission wavelength band of the first opticalfilter and the output spectrum of the semiconductor laser.
 15. A pulsecompression method according to claim 14, wherein the first opticalfilter outputs the attenuated pulse light after attenuating the opticalpower on the shorter wavelength side more than that on the longerwavelength side of the wavelength band of the inputted pulse light. 16.A pulse compression method according to claim 14, wherein the firstoptical filter has variable transmission characteristics.
 17. A pulsecompression method according to claim 14, further comprising the step ofdisposing a second optical filter so as to sandwich an opticalamplification medium included in the optical amplifier with the firstoptical filter.
 18. A pulse compression method according to claim 14,wherein the first optical filter includes a band-pass filter.
 19. Apulse compression method according to claim 17, wherein at least one ofthe first optical filter and the second optical filter includes aband-pass filter.
 20. A pulse compression method according to claim 17,wherein each of the first optical filter and the second optical filterincludes a band-pass filter, and wherein a full width at half maximum ofa transmission spectrum in the second optical filter is wider than afull width at half maximum of a transmission spectrum in the firstoptical filter.
 21. A pulse light source according to claim 17, whereineach of the first optical filter and the second optical filter includesa band-pass filter, and wherein a center wavelength in a transmissionspectrum of the second optical filter is set between the peak wavelengthof the pulse light and a center wavelength in a transmission spectrum ofthe first optical filter.
 22. A pulse compression method according toclaim 14, wherein the semiconductor laser includes a Fabry-Perot typesemiconductor laser.
 23. A pulse compression method according to claim14, further comprising the step of providing a temperature adjustmentunit adjusting temperature of the semiconductor laser.